1. Field of the Invention
The field of the invention is that of systems for monitoring battery aging. The invention applies in particular to monitoring the aging of backup batteries for optical repeaters, for example. Monitoring of aging checking whether a battery is able to meet a requirement or not, in particular to provide a backup power supply.
The invention also concerns a method for monitoring battery aging. In the remainder of this description the term battery refers to a set of rechargeable cells connected in series.
2. Description of the Prior Art
Backup batteries are frequently used in optical repeaters of optical fiber communication systems. The batteries supply power to a repeater if the main power supply to the repeater derived from the AC line voltage, for example, fails, (i.e., a temporary AC line power outage).
To monitor the aging of backup power supplies two identical batteries are usually connected in parallel by switches, as shown in FIG. 1.
Two batteries 10, 11 are in a circuit comprising two branches. Each battery 10, 11 cooperates with a power switch 12, 13 connected to a load 16 to be supplied with power and to means 14 for measuring the aging of each battery. The batteries 10 and 11 are to supply power to the load 16 if absence or failure of the main power supply of the load 16 is detected. If such failure occurs, the switches 12 and 13 are operated so that the load 16 is connected to the two batteries 10 and 11 in parallel.
The aging of a battery is estimated by operating the associated switch, for example the switch 12 for the battery 10, and causing the battery to discharge current into the measuring means 14 which include means for discharging the battery connected to a voltage reference Uref representing a given percentage, for example 50%, of the nominal capacity of a charged battery.
FIG. 2 shows various battery voltage characteristics as a function of the state of charge (S.C.) expressed as a percentage.
The characteristics 20 through 22 are for lead-acid batteries. Characteristic 20 is for a brand new lead-acid battery, characteristic 21 for the same battery after use for a certain period and characteristic 22 for the same battery after use for a long period. If the voltage Uref is set at 50% of the nominal capacity of a battery functioning perfectly, discharging a used battery of the same type to Uref does not provide the same capacity (quantity of electricity). The residual capacity can be determined by measuring the energy supplied by the battery under test until the voltage across it reaches Uref.
For example, a brand new 10 Ah battery may have a capacity of only 5 Ah after ten years and it is therefore necessary to know how it has aged in order to ensure that the backup power supply system operates correctly.
To this end the measuring means 14 include means for measuring the capacity (in Ah) output by the battery under test from its maximally charged state to the reference voltage Uref. If the battery under test no longer delivers sufficient capacity, it is deemed to be too old and the module 14 reports this to an operator who can then replace it. This reporting is usually done via a transmission bus 15 which provides a remote maintenance link for checking the condition of the batteries.
If the battery is still operational, i.e. if the capacity output when it is discharged is sufficient, the measuring means 14 recharge it before testing the other battery.
The main drawback of this type of system for monitoring battery aging is that it requires two batteries because the backup power supply provided by the two batteries may be required at any time. In some cases only 75% of the total capacity is available, i.e. when one of the batteries is discharged 50% (at the end of the cycle for checking its charge) and the other is fully charged. Increasing the number of batteries is not an acceptable solution because this increases the cost of the backup power supply system and it is necessary to provide a greater number of power switches, one per battery, and these switches are costly if the current drawn by the device is high.
The batteries used in this type of backup power supply are lead-acid batteries and in this case further discharging a battery already discharged 50% may destroy it. This can occur when, at the end of discharging a battery in order to test its residual capacity, the backup power supply is required to supply power to the load 16.
Also, lead-acid batteries have electrochemical properties which fall well short of those of more recent battery types such as nickel-cadmium batteries. Nickel-cadmium batteries have a characteristic like that of line 23 in FIG. 2 (i.e. the battery voltage remains substantially constant throughout discharge). This means that it is not possible to detect when the voltage across these batteries falls below the voltage Uref without entirely discharging the battery.
The use of two nickel-cadmium backup batteries, whose aging is tested as shown in FIG. 1, therefore leads to a remaining overall capacity of only 50% on completion of testing one battery. If the backup power supply is required immediately following failure of the main power supply, this residual capacity of 50% may be insufficient. To retain the 75% capacity obtainable with lead-acid batteries it is therefore necessary to use four nickel-cadmium type batteries and therefore to increase the number of switches.
One specific object of the present invention is to alleviate these drawbacks.
To be more precise, one object of the invention is to provide a system for monitoring the aging of a backup power supply using only one battery.
Another object of the invention is to enable the use of batteries which can be fully discharged, such as nickel-cadmium, nickel-hydride or lithium-carbon batteries, which increases the capacity of the backup power supply.